The manual trigger, as its name suggests, allows you to fire rules manually and instantly rather than having to wait for a scheduled point in time or for a particular event to occur. After a rule is fired manually, the rule execution process follows all the steps already described in the Rules section - evaluation of a condition and if the condition is satisfied, then triggering of the rule and launching of a task.
Usually, the manual trigger is combined with other types of triggers in a single rule, i.e. with device event and cron triggers.
Consider for example a rule whose action is such that affects only devices which are currently online - such as sending a command to the device, which is impossible if there is no connection with it. In that case the manual trigger (if the condition is satisfied) will launch a task which will be executed only on those devices in the rule scope which are currently online. In order to cover the rest of the devices in the scope - those currently offline, you need to add a device event trigger with a DeviceOnlineEvent, which will launch a new task for each device of the scope when it becomes online.